Caffeine Half-Life & Sleep

With a half-life of about 5–6 hours (longer in slow CYP1A2 metabolisers and in people on oral contraceptives), a 200 mg espresso at 4 PM leaves roughly 100 mg circulating at 10 PM. Caffeine blocks adenosine, the sleep-pressure signal, and reliably reduces deep sleep even when people report falling asleep normally. A practical rule is a hard cutoff 8–10 hours before bed, and delaying the first coffee 60–90 minutes after waking to blunt the afternoon crash.